A cardiac defect that causes oxygenated blood from the lungs to be shunted back into the pulmonary circulation. Leads to pulmonary hypertension and right ventricular pressure overload, which causes right-sided heart hypertrophy and eventually heart failure without cyanosis. Etiologies include septal defects and a patent ductus arteriosus. If untreated, can progress to a right-to-left shunt due to worsening pulmonary hypertension.